(1) Field of the Invention
The present invention relates to a paper feeding device for copiers, and more particularly relates to an increased paper feeding device of frictionally separating roller mechanism.
(2) Description of the Related Art
As typical paper feeding devices of frictionally separating roller mechanism, there has been known one which is constructed as shown in FIG. 1. Specifically, a conventional paper feeding device of frictionally separating roller mechanism comprises: a pick-up roller 2 which presses the top surface of a paper sheet stack 1 and delivers an upper most sheet with frictional force to a nip defined between a feed roller 3 (to be referred to later) and a reverse roller 6 (to be referred to later); a paper feed roller 3 for feeding with frictional force the sheet delivered by the pick-up roller 2 in a direction of its rotation; and a reverse roller which is driven by means of a torque limiter 4 in a direction opposite to that of the paper feed roller 3 and is pressed against the paper feed roller 3 with a braking force acted by a pressure spring 5 and a braking pressure caused by a torque of the torque limiter 4. A lot of examples of such a paper feeding device using a torque limiter are known, and an example is shown in Japanese Utility Model Application Laid-Open Sho 59 No. 187647.
The rollers mentioned above generally use rubber molds made of essentially polynorbornene rubber and/or rubber molds made of essentially urethane rubber. When the paper feed roller 3 and reverse roller 6 are formed of the same material, a torque limiter 4 must be provided to the reverse roller 6. The reverse roller 6 is driven by means of the torque limiter 4 in a rotational direction opposite to the proceeding direction of sheets while the roller 6 is pressed against the paper feed roller 3 with an initial pressure acted by the pressure spring 5 and acting pressure caused by the torque of the torque limiter 4. In this arrangement, when the reverser roller 6 is in direct contact with the paper feed roller 3 or a sheet of paper is fed into the intake nip, the reverse roller 6 receives a torque beyond the limit torque and runs idle so as to be driven by the paper feed roller 3. In contrast, if two or more sheets unseparated are fed into the nip, the limit torque surpasses the frictional force between the sheets and causes the reverse roller 6 to rotate in the paper reversing direction so as to push back the lower sheet, preventing the plural sheet feeding.
On the other hand, there have been known paper feeding devices of frictionally separating roller mechanism, in which, taking advantage that urethane rubber has a smaller friction coefficient than polynorbornene, the paper feed roller 3 is formed with polynorbornene rubber and the reverse roller 6 is formed with a urethane rubber in stead of forming the paper feed roller 3 and reverse roller 6 with a like material. A paper feeding device of this type prevents the plural sheet feeding not employing a torque limiter 4 but using the difference of friction coefficient to sheets between the paper feed roller 3 and the reverse roller 6.
Nevertheless, in the paper feeding devices of frictionally separating mechanism as described above, particularly, for example, in the paper feeding device with the paper feed roller 3 and reverse roller 6 made with the same material, a sheet of paper delivered will be conveyed by a differential force between a frictional force F.sub.1 caused by the paper feed roller 3 and another frictional force F.sub.2 caused by the reverse roller 6 as shown in FIG. 2(a) (refer to FIG. 3(a)). When two sheets piled are delivered, the lower side sheet is separated from the upper one by a differential force between a frictional force F.sub.2 caused by the torque limiter 6 and another frictional force F.sub.3 generated between the sheets as shown in FIG. 2(b) (refer to FIG. 3(b)). Therefore, an appropriate pressing force is to be determined dependent upon the specification of the torque limiter (f.sub.1 and f.sub.2 in FIG. 3), but the range is considerably limited.
On the other hand, when used is a paper feeding device with materials of the paper feed roller and the reverse roller being different (for instance, a paper feed roller of polynorbornene rubber, and a reverse roller of urethane rubber), an appropriate pressing force is hard to select because either material has a different dependence of its friction coefficient upon the quality of paper to be used, and degradation of rubbers due to environmental conditions such as temperature, humidity, etc.
For example, in a case of using an OHP sheet (a sheet for overhead projector), the relation between the friction coefficients of urethane rubber and polynorbornene rubber is reversed (the friction coefficient of urethane rubber is larger). As a result, the situation becomes more complicated.